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La Niña may be on its way, says the UN's weather agency, an event that tends to bring above-average rainfall to Australia and parts of Southeast Asia.

The World Meteorological Organization (WMO) says it has seen unprecedented signs pointing to La Niña, which originates off the western coast of South America but can disrupt weather patterns around the globe.

The Geneva-based agency says temperatures in the central and eastern equatorial Pacific have been between 0.5 and 1.0°C below normal since the start of 2006.

"Combined with broader tropical Pacific Ocean and atmosphere conditions, this is consistent with the early stages of a basin-wide La Niña event," it says.

"It is unprecedented in the historical record for a La Niña of substantial intensity or duration to develop so early in the year."

La Niña, which has the opposite effects to the more notorious El Niño, last occurred from mid-1998 to early 2001.

Under La Niña, the sea-surface temperature in the central and eastern tropical Pacific falls below normal.

This typically brings far dryer weather to the southwestern US, Florida and western Latin America and above-average rainfall to Australia, Indonesia, Malaysia and the Philippines.

But there can also be a knock-on effect much further afield, with an increase to monsoon rainfall in South Asia, unusual coolness in tropical West Africa, Southeast Africa, Japan and the Korean peninsula.

The build-up of this La Niña is so exceptionally swift and intense that it is impossible at the moment to infer what the impact would be, and how long the phenomenon would last, it warns.

"Most models and expert interpretations favour the event dissipating quite rapidly over the next three to six months," it says.

"Nonetheless, neither a continuation of La Niña beyond mid-year, nor the development of El Niño in the second half of 2006, can be ruled out as possible outcomes from the current prevailing situation."

El Niño, which last ran from 2002 to 2003, occurs when warm water builds up in the western tropical Pacific and creeps eastwards, again causing huge disruption to classic patterns of rainfall and wind.

Both El Niño and La Niña are naturally occurring cycles, although there is much speculation among climate scientists that human induced global warming may make them more frequent and more vicious and that this trend may have already started.

El Niño means 'the little boy' in Spanish. Its name is attributed to fishermen off the coast of South America who noted the appearance of warmer water, often around Christmas. La Niña means 'the little girl'.